a. Field of the Invention
This invention relates to a velocity control apparatus for controlling the velocity of a servomotor by PWM (pulse-width modulation) control of an inverter.
b. Description of the Related Art
In recent years, microprocessors have come to be widely used in velocity control apparatuses for controlling the driving of the servomotor. With a motor drive circuit composed of digital circuitry using a microprocessor, table information such as amplitude characteristics decided in conformity with motor characteristics is created, and the velocity of a servomotor is controlled by PWM control of an inverter based on a set velocity signal.
In a conventional velocity control apparatus of this kind, the range of fluctuation allowed with regard to power supply voltage is set to be within a prescribed limit. The apparatus is provided with a function which protects the motor from overcurrents when the aforementioned limit is exceeded, and with a protective function which shuts down the system automatically when the voltage is too low. However, no particular consideration is given to fluctuations within the prescribed limit.
More specifically, a fixed current loop constant K is provided. Therefore, when power supply voltage Vdc becomes high, current loop gain K' (=K.multidot.Vdc/Pa, where Pa represents carrier wave amplitude) rises in proportion to the voltage value and there is the danger that the motor current will oscillate. Conversely, when the power supply voltage becomes low, current loop gain diminishes and the response of the velocity loop to a commanded velocity signal slows down. The result is a deterioration in the quick response of the control system.